1. Field of the Invention
This invention relates to a branching device to process high-frequency signals in a plurality of frequency bands.
2. Description of the Related Art
In recent years, as a technique allowing a network to be easily established, a wireless LAN in which a LAN is established using radio waves has been developed. In the wireless LAN, there are a plurality of standards, such as IEEE802.11b, in which a 2.4 GHz band is used as a frequency band and IEEE802.11a in which a 5 GHz band is used as a frequency band. Therefore, a communication device used for the wireless LAN incorporates a branching device that is connected to an antenna and processes high-frequency signals in a plurality of frequency bands.
For example, in Japanese Unexamined Patent Application Publication No. 2006-352532, a branching device (high-frequency module) is disclosed that includes a first diplexer on a receiving side, a second diplexer on a transmitting side, and one switch circuit. In this branching device, the switch circuit switches to and connects one of the first and second diplexers to one antenna. The first diplexer on the receiving side includes a first band pass filter allowing a high-frequency signal within a predetermined frequency band to pass therethrough. The second diplexer on the receiving side includes a second band pass filter allowing a high-frequency signal within a predetermined frequency band to pass therethrough. The first and second band pass filters are configured in a laminated substrate in which a dielectric layer on which a conductor pattern is formed is laminated.
FIG. 1A is a plan view illustrating the top surface of a twelfth dielectric layer 612 in the laminated substrate in Japanese Unexamined Patent Application Publication No. 2006-352532, FIG. 1B is a plan view illustrating the top surface of a thirteenth dielectric layer 613 in the laminated substrate in Japanese Unexamined Patent Application Publication No. 2006-352532, and FIG. 1C a plan view illustrating the top surface of a fourteenth dielectric layer 614 in the laminated substrate in Japanese Unexamined Patent Application Publication No. 2006-352532.
A conductor layer 739 illustrated in FIG. 1A is connected to a conductor layer 740 illustrated in FIG. 1B through a via hole. In addition, the conductor layer 740 illustrated in FIG. 1B is also connected to a conductor layer 741 illustrated in FIG. 1C through a via hole. Accordingly, the conductor layers 739 to 741 illustrated in FIG. 1A to FIG. 1C define an inductor L7 included in the first band pass filter on the receiving side.
In the same manner, a conductor layer 839 illustrated in FIG. 1A is connected to a conductor layer 840 illustrated in FIG. 1B through a via hole. In addition, the conductor layer 840 illustrated in FIG. 1B is also connected to a conductor layer 841 illustrated in FIG. 1C through a via hole. Accordingly, the conductor layers 839 to 841 illustrated in FIG. 1A to FIG. 1C define an inductor L8 included in the second band pass filter on the transmitting side.
However, when a high-frequency signal passes through the first band pass filter on the receiving side, a magnetic field occurs in the inductor L7. Therefore, the magnetic field interferes with the inductor L8 in the second band pass filter. In the same manner, when a high-frequency signal passes through the second band pass filter on the transmitting side, a magnetic field occurs in the inductor L8. Therefore, the magnetic field interferes with the inductor L7 in the first band pass filter. In particular, in Japanese Unexamined Patent Application Publication No. 2006-352532, since the inductor L7 and the inductor L8 are disposed on a same dielectric layer, unnecessary interference due to mutual magnetic fields may easily occur.
Therefore, in a branching device, it is important for an isolation characteristic between individual LC filter circuits to be improved so that signals passing through the individual LC filter circuits do not cause unnecessary interference with each other (in particular, between transmission and reception). Specifically, in Japanese Unexamined Patent Application Publication No. 2006-352532, in order to reduce electromagnetic interference between the inductors L7 and L8, via electrodes V connected to ground terminals G1 and G4 through a conductor layer 652 are provided between the inductors L7 and L8. However, in this case, a sufficient space to provide the via electrodes V is required between the inductors L7 and L8.
Therefore, in the laminated structure of the branching device in Japanese Unexamined Patent Application Publication No. 2006-352532, there is a problem in that downsizing is difficult while also reducing the electromagnetic interference. Accordingly, when a mobile communication device, such as a mobile phone, includes the branching device according to Japanese Unexamined Patent Application Publication No. 2006-352532, a problem occurs in that the size of the mobile communication device is increased.